Download Click Here To File

PRE-BOARD-II EXAMINATION SESSION 2016-17
Model Answers
SUBJECT: CHEMISTRY (043)
Q.NO.
1
2
ANSWERS
The trapping of anionic sites by the electrons in metal excess non-stoichiometric crystal
defects are called F-centres.
[Fe(OH)3]Fe3+….. is a positively charged sol hence it will be neutralized by the negative
part of the an electrolyte. According to Hardy-Schulze rule the flocculation power is
directly proportional to charge density of the ion.
𝑎𝑞
3
K4[Fe(CN)6]→ 4K+ + [Fe(CN)6]4- (aq)
3 (a) ,because 2 – chlorobutane when undergoes SN1 reaction , produces 20 carbocation
whereas 1-chlorobutane gives 10carbocation. Since 20 carbocation is more stable than 10
carbocation, therefore 2-chlorobutane undergoes SN1 reaction faster than 1- chlobutane.
4
2-Acetoxy benzoic acid.
5
C2H5NH2 < (C2H5)3N < (C2H5)NH
6
a) The reaction at cathode in a dry cell :
MnO2+ NH4+ + e- →MnO(OH) + NH3
ZnCl2 combines with NH3 produced to form complex [Zn(NH3)2Cl2], otherwise the pressure
developed due to NH3 would crack the seal of the cell.
𝑎𝑞
b) AgNO3→ Ag+ (aq) + NO3- (aq)
H2O<------ H+ +OHAt cathode: Ag+ + e- → Ag(s) discharge potential of Ag+ is greater than H+
Ag+ ions will be deposited as silver.
At anode: Ag(s)→ Ag+ + eAg electrode is attacked by NO3- ions, Ag anode will dissolve to form Ag+ ions in the
solution.
7
a)(i)The sum of powers of the concentration of the reactants in the rate law expression is
called the order of reaction whereas the number of reacting species (atoms, ions or
molecules) taking part in an elementary reaction, which must collide simultaneously in
order to bring about a chemical reaction is called molecularity.
(ii) Order is experimental whereas molecularity is theoretical
(iii) Order may be zero or fractional but molecularityis always integral
eg CH3CHO → CH4 + CO
Rate = K[CH3CHO]3/2
Order= 3/2
Molecularity= 1 .(or any other example).
b) Inversion of cane sugar/ hydrolysis of ester/ SN1reaction .(or any other example).
eg Ester + H2O → Acid + Alcohol
8
Rate = K[Ester]1[Water]0
Order = 1 ,Seems to follow 2nd order kinetics but actually follows 1st order kinetics due to
its independence to the conc. of water which is taken in large excess
(i) Preparation of Sodium Chromate
4FeO.Cr2O3 + 8Na2CO3+7O2 →8Na2CrO4+2Fe2O3+8CO2
(ii) Conversion of Sodium Chromate into Sodium
Dichromate
2Na2CrO4+ H2SO4 →Na2Cr2O7+ Na2SO4+ H2O
(iii) Conversion of Sodium Dichromate into Potassium Dichromate
Na2Cr2O7 + 2KCl → K2Cr2O7+ 2NaCl
Effect of increasing pH on aqueous solution of K2Cr2O7
Cr2O72- + H2O → 2CrO42- + 2H+
←
OR
9
10
(i) Electronic Configuration – The general electronic configuration of lanthanoids is [Xe]54
4f1-14 5d0-1 6s2 whereas that of actinoids is
[Rn]54 5f1-14 6d0-1 7s2
(
1) (ii) Oxidation State – Lanthanoids show limited oxidation states ( +2,+3, +4)
but +3 is the common oxidation state whereas actinoids show large
number of oxidation states (+3, +4, +5, +6, +7).
(i) The major product is (CH3)2C=CH2 , C2H5OH and NaBr
(ii) (CH3)3CO-1Na++ C2H5Br → (CH3)3COC2H5 + NaBr
Mechanism of hydration of ethene.
Step I : Protonation of Alkene
CH2 = CH2 + H+ ⇋ CH3 - +CH2
Step II : Nucleophile attacks on carbocation
CH3 - +CH2
Step III : Deprotonation
CH3 - CH2 - +OH2
11
..
H2O
..
..
CH3 - CH2 - +OH2
Protonated Alchohol
H+
For fcc structure, z= 4
a= 4.07 x 10-8cm
d=10.5gm/cc
N0= 6.022 x 1023
d= Z x M/a3 x N0
M= d x a3 x N0/z
= 10.5 X (4.07)3 x10-24 x 6.022 x 1023/4
= 106.55g
CH3-CH2-OH
Half cell reaction
At Anode: Zn → Zn2+ + 2eAt cathode: 2H+ + 2e- → H2/Pt
______________________________
Zn(s) + 2H+(10-2M) →Zn2+(10-3M) + H2/Pt
E 0Zn2+/Zn = -0.76V
12
E 0 H+/H2= 0.00V
E cell =E 0cell- 0.059/n log [Zn2+] / [H+]2
Substituting values
E cell = 0.76 – 0.0591/2 log 10-3/10-4
E cell = 0.76 – 0.0295 log 10 (log 10=1)
= 0.7305 V
13
14
SO2Cl2 (g) → SO2(g)+ Cl2(g)
k = (2.303/t) log p0/(2p0-pt)
t= 100s
k= (2.303/100) log 0.5/2 x 0.5- 0.6
=2.303/100 x log1.25
= 2.303/100 x 0.0969 = 2.23 x 10-3 s-1
When Pt = p0 + p = 0.65 atm , p = 0.65 – p0 = 0.65 – 0.50 = 0.15 atm
Pressure of SO2Cl2= p0 - p =50- 0.15 = 0.35 atm
Rate =k x P SO2Cl2= 2.2316x 10-3 x 0.35 = 7.8 x 10-5atms-1
(a) Breaking of emulsions by heating, freezing, centrifugating etc.
Examples - Epoxy resins, poly amines, polyols, siloxanes etc.
(b) The catalytic reaction that depends on the pore structure of catalyst and size of
reactant and product molecules is called shape selective catalysis.
𝑍𝑆𝑀−5
15
16
Eg alcohols →
gasoline
(a) In froth floatation process, the role of the depressant is to prevent one type of
sulphide ore particles from forming thr froth with air bubbles. For example,
NaCN is used as a depressant to separate lead sulphide (PbS) ore from zinc
sulphide (ZnS) ore. Because NaCN forms a complex with ZnS, Na2[Zn(CN)4], but
PbS forms froth.
(b) The standard free energy of formation of CO2 from CO is higher than that of
formation of ZnO from Zn.
(c) The impurities are more soluble in the molten state than in the solid state of the metal.
(a) P4+3NaOH+3H2O → PH3 + 3Na2H2PO2
(b) 2Fe3+ + SO2 +2H2O →2Fe2++ SO42- +4H+
(c) 4NaCl + MnO2 +4H2SO4 → MnCl2 +4NaHSO4 +2H2O +Cl2
17
(a) (i) Oxygen stabilizes the highest oxidation state even more than fluorine, e.g,
highest fluoride of Mn is MnF4 whereas highest oxide is Mn2O7. The reason for
this is the ability of oxygen to form multiple bonds with metal atoms
(ii) Transition metals and their compounds show catalytic properties because of:
. Variable oxidation state
. Unpaired electrons in their incomplete d – orbitals
. Large surface area
(b) 3MnO42- + 4H+ → 2MnO4-1 + MnO2 + 2H2O
(a)Correct structure of cis and trans isomer
(b) t2g4 eg0
(c) dsp2, diamagnetic
18
OR
[Ni(CO)4] – tetrahedral geometry, diamagnetic, sp hybridisation
[CoF6]3- - octahedral geometry, paramagnetic, sp3d2hybridisation
[Co(NH3)6]3+ - octahedral geometry, diamagnetic, d2sp3hybridisation
3
. a) CH3 – CH = CH2 + HBr
CH3CH2NO2
19
b) CH3 – CH2 – OH
c)
Br2
FeCl
2NaBr
3
CH3CHCH2Br
SOCl
2
CH3CH2Cl
+2 Na
CH3CH2F
+
AgF
Fitting Rxn
(dryether)
Biphenyl
A) Aniline doesn’t undergoes friedel crafts reaction as aniline is a Lewis base it reacts with
catalyst AlCl3 which is Lewis acid & forms a complex which deactivates the benzene ring
towards electrophilic substitutions reactions.
20
B) i) CH3CONH2+Br2+4NaOH→ CH3 – NH2 + 2NaBr +Na2CO3 + H2O
(Hoffmann Bromamide Degradation Reaction)
ii) C6H5N2Cl + C6H5OH
(Coupling Reaction)
22
C6H – N = N – C6H5OH + HCl
( Orange Azodye)
a) Tranquilizers are chemical compounds which are used to cure stress, anxiety, mild or
severe mental diseases For eg: Derivatives of Barbiturates, luminal, equanil etc.
b) The use of aspartame is limited to cold food and drinks because it decomposes on
heating and loses its sweetening properties.
c) Antidepressant drugs should not be taken without consulting a doctor as it may increase
aggression, anxiety, and can cause hypertention.
23
a) Fat soluble vitamins
b) Concern for health, awareness about vitamins, helpful.
c) Vitamin B12 can be stored in our body.
d) Deficiency of Vitamin A – Night Blindness
Deficiency of Vitamin D – Rickets
(a)As solubility of gases decreases with increase of temperature, less oxygen is available in
summer in the lakes / as cold water contains more oxygen dissolved.
(b)0.91% saline water is isotonic to human blood cells. When placed in a hypertonic
solution (conc.> 0.91% NaClsoln) they will shrink due to exo-osmosis.
(c) P0- Ps / P0= XB
P 0 - P s / P 0 = W B x MA / M B x W A
1-Ps/P0 = 15 x 18/180 x 150
24
= 1/100
Ps/P0 = 1- 1/100
Ps/P0 = 99/100
Ps = P0 x 99/100
= 17.5 x99/100 mm Hg
= 17.325 mm Hg
(i) Nitrogen shows less tendency for catenation than phosphorous because of its
small size and more repulsion due to lone pair of electrons on the two N – atoms
whereas in phosphorous there is less repulsion on P –P bond due to its
comparatively larger size.
(ii) Due to presence of two unpaired electrons in the π*antibonding orbitalof S2
molecule.
(1)
(iii) Bond dissociation enthalpy of fluorine is less than chlorine because of small size
of fluorine atom, there is more repulsion between lone pair of electrons of F- atoms
25
(b) (i) XeF6 + 3H2O → XeO3 + 6HF
(1)
(ii) 4H3PO3 → 3H3PO4 + PH3
(1)
OR
(a) Any of the two poisonous gases like phosgene (COCl2), tear gas (CCl3NO2), mustard
gas (ClCH2CH2SCH2CH2Cl), etc.
(b) (i) High pressure
(ii) low temperature
(iii) use of catalyst
(c) When O3is treated with excess of KI solution buffered with borate buffer, I2 is
liberated.
2I- (aq) + H2O (l) + O3 (g) → 2OH- (aq) +I2 (s) + O2 (g)
I2 so liberated is titrated against standard solution of sodium thiosulphate using
starch as indicator.(1)
(d) Correct structure
(e) Halogens are colored because there molecules absorb light in the visible region
which excite their electrons to the higher energy levels whereas the remaining light
is transmitted. The color of halogens is the color of its transmitted light that is
halogens have complementary colors. The amount of energy required for excitation
decreases from fluorine to iodine as the size of the atom increases. Conversely , the
energy of transmitted light increases progressively i.e. deepening of colour of
halogens from fluorine to iodine.
a)
i)
Aldol Condensation
CH3CHO + CH3CHO
NaOH
H H
CH3–C–C–CHO
OH H
- H2O
CH3–CH=CHCHO
But-2enal
ii)
Cannizzaro Reaction
26
HCHO + HCHO
KOH
CH3OH + HCOOK
b)
i)
CH3COCH3 and CH3CHO
These can be distinguished by Fehling solution test or Tollen’s Regent test. CH3CHO will
give silver mirror with Tollen’s Reagent or red ppt with Fehling solution.
ii)
Ethanal and Ethanoic acid can be distinguished by Tollen’s reagent test or
NaHCO3 test Ethanoic acid will give brisk effervscences with NaHCO3 due to
evolution of CO2 gas.
c) In 4 Nitrobenzoic acid NO2 group has -I effect so it decrease e- density on benzene and
hence removal of H+ becomes easier and is more acidic whereas in 4-Methyl benzoic acid
methoxy gp has +I effect and increase e- density on benzene and hence removal of H+
becomes difficult making it less acidic.